There is little known concerning the incidence and risk factors for adolescent injuries. What has been identified so far is that the highest risk period of sports and recreational injuries is the teenage years where there is an apparent "epidemic" with one in ten children per year being seen at a hospital. The proposed research is a prospective study of 1,400 seventh, eighth, and ninth graders in a single defined school district outside of Pittsburgh. We will collect indepth assessments of activity levels at baseline and then follow the cohort for 3 years. In addition to activity levels, potential cofactors including gait, physical fitness and behavioral factors will be assessed. During the three-year period, 300 injuries sufficiently severe to seek hospital attention are predicted to occur. The prospective design of the research thus permits us to determine: 1) the incidence of serious teenage injury overall and specifically related to sports; and 2) the relationship of activity level to risk.